Present copper recovery processes utilize copper smelters which inherently create a significant amount of air pollution and hence ecological harm to the environment. In recent years, there has been strong emphasis to minimize ecological damage to the environment and as a consequence certain processes have been developed for the treatment of copper concentrates to recover copper without attendant environmental problems.
U.S. Pat. No. 3,901,776, Kruesi et al., Aug. 26, 1975, discloses a copper electrowinning process which purports to separate the anolyte and catholyte compartments through the use of a microporous polypropylene film. It is doubtful that a film of the type disclosed by Kruesi et al. performs effectively because such a film is a physical barrier, rather than a chemical complex barrier, and has very small openings (of the order of 0.02 microns) through it. Such a film, since it is a diaphragm and not a true membrane, does not achieve effective separation because it allows solution and ions to pass through it, albeit at a very slow rate.
A consideration of the composition of the solution in the Kruesi et al. patent indicates plainly that solution exchange between compartments is necessary in order to allow copper electrowinning to proceed. The solution from leach 1 (see column 8) after reduction with recycled copper would contain 67.9 gpl Cu, or just over 1 molar Cu. In order to remove this copper from solution, one mole of ions must pass through the film, either as cations moving from the anolyte to the catholyte, or as anions moving from the catholyte to the anolyte. The solution is indicated to be pH =0.5 which corresponds to approximately 0.3 molar HCl. If 0.3 moles of protons pass from the anolyte to the catholyte, then it follows that 0.7 moles of charge remain to be passed to maintain a charge balance. Some of this current must necessarily be passed by the transfer of chloride ions from the catholyte to the anolyte.
The applicants have tested a film of the type disclosed by Kruesi et al. and have noted that chlorine ion transfer through the film takes place. With solutions as described by Kruesi et al. and having the anolyte and catholyte compartments separated by a CELGARD (trade mark) film, a film of the type used by Kruesi et al., the applicants have observed that after operating an electrowinning cell with such a film for only 100 hours, the polypropylene film becomes embrittled due to conversion of the polypropylene to a chlorinated polypropylene structure.
In addition to charge transfer by proton (H.sup.+) and chloride ion transport, the applicants have noted that in a process of the type disclosed by Kruesi et al., additional charge transfer will occur due to the diffusion of copper and iron ions through the separator.
A hydrometallurgical process for the electrolytic recovery of selected base metals (especially copper, and optionally nickel) from sulfide ore concentrates concurrently with the extraction of metallic iron in commercially usable quantities had previously been developed by the applicant and is now the subject of U.S. Pat. No. 4,159,232, issued June 26, 1979. The process disclosed and claimed in that patent utilizes at least a primary and a secondary bank of sequentially disposed electrolytic cells, the cells of each bank being electrically connected in parallel. Each of the cells has separate anode and cathode compartments, the compartments being separated from other compartments in a bank by a permeable dividing element capable of passing electrolyte between the compartments.
A first supply of anode solution is continuously withdrawn from the anode compartments of the electrolytic cells in each bank of cells, the anode solution being an aqueous electrolyte including in solution hydrochloric acid and a soluble metal chloride. The anode solution is transported to at least one leaching vessel to continuously leach a supply of ore concentrate to one leaching vessel to reduce ferric ions in solution to their lowest valence state (ferrous). The leaching vessel generates a liquid-solid slurry output including solid residue, partially leached concentrate and leaching solution. The solids are separated from the liquid in the resulting slurry output by using a suitable solids-liquid separator. The solution from the solids-liquid separator is returned to the cathode compartments of both the primary and secondary banks of cells, with a first preselected portion of the liquid being returned to the cathode compartments of the secondary bank of cells.
Base metal is precipitated at the cathode in a non-adherent form to provide a slurry with a cathode solution. Base metal deposits are continuously withdrawn from the slurry comprising cathode solution and precipitated base metal obtained from the cathode compartments of the bank of cells. The separated cathode solution obtained from the base metal slurry is returned to the cathode compartments of both the primary and secondary bank of cells with a second pre-selected portion of the separated cathode solution being returned to the cathode compartments of the secondary bank of cell. The amounts of the first and second pre-selected portions are small enough to allow for the establishment in the cathode compartments of base metal impoverished solution areas.
The amounts of remaining separated leaching liquid and separated cathode solution returned to the cathode compartments of the primary bank of cells are large enough to avoid the development of base metal impoverished areas next to the cathode electrodes of the primary bank of cells. Base metal impoverished cathode solution is continuously withdrawn from the base metal impoverished solution areas of the cathode compartments of the secondary bank of cells. Hydrogen gas is cathodically produced from the catholyte of the electrolytic cells in each bank of cells. The base metal impoverished cathode solution is evaporated and crystallized to yield hydrated ferrous chloride. The hydrated ferrous chloride is reduced by the hydrogen gas at a selected elevated temperature to produce metallic iron. Copper is obtained from the cathode compartments of both . the primary and secondary banks of cells.
In this process, it is important to note that mixing of the anolyte and the catholyte takes place across the permeable dividing element.